This invention relates to composite optical elements, optical isolators, optical circulators, and optical switches for applications in optical communications and measurements, and also to processes for producing the same.
In Japanese Patent Application Kokai No. 5-181088 the present inventors proposed a novel, polarization-insensitive optical isolator comprising a Faraday rotator positioned between a pair of birefringent diffraction grating elements. Known birefringent diffraction grating elements useful for the optical isolator are as follows. They are all diffraction grating polarizers. Each element of the present invention by itself does not function as, but is technically equivalent to, a polarizer.
(1) The element according to Japanese Patent Application Kokai No. 63-55501; an element in which a diffraction grating is formed by subjection of lithium niobate to proton ion exchange. The element has a problem of high-cost manufacture because of the expensive single crystal substrate of lithium niobate for the grating. Another problem is that the difficulty of precise control of the optical path difference for polarization obstructs the fabrication of the elements in a stable way with good reproducibility.
(2) The element according to Japanese Patent Application Kokai No. 2-156205; a polarizer having a dielectric layer at the bottoms of grooves formed at regular intervals on the principal surface of an optically anisotropic crystal plate. The polarizer can be made at low cost, but the difficulties in accurately controlling the depth of grooves and the thickness of the dielectric layer render it impossible to control the optical path difference for polarization with high precision. Consequently, as with (1), stable production with good reproducibility has not been attained. In addition, rough bottom surface of the grooves can cause scattering, leading to deteriorated characteristics.
The present invention, therefore, aims at solving the afore-described problems and providing diffraction grating elements, especially birefringent diffraction grating elements, which permit easy control of the optical path length, production scheme, and designing, exhibit high performance stability with time, and have reduced thickness and also providing processes for producing them, and further providing optical isolators, optical circulators, and optical switches using those elements, and processes for producing them.
The composite optical element in one aspect of the present invention comprises a first optical material and a second optical material joined to one plane of a third optical material, the first and second optical materials being the same in thickness and having ground principal planes flush with each other on both sides of them. To be more concrete, either the first or second optical material is a birefringent material. Alternatively, both of the first and second optical materials are birefringent and have functions as wave plates. The optical element has a broad range of applications as a component member for polarizers, diffraction gratings, optical isolators, optical circulators, and optical switches.
The composite optical element of the structure described is produced by the process according to of the invention. The process is a process for producing a composite optical element comprising the steps of:
forming a plurality of first grooves at predetermined intervals in a first optical material;
forming a plurality of second grooves at predetermined intervals in a second optical material;
bonding the first optical material having the first grooves and the second optical material having the second grooves together, with their grooves and lands staggered to fit each other, through an adhesive to provide a composite block;
grinding one side of the composite block to a thickness where both surfaces of the first and second optical materials are exposed;
bonding a third optical material, with a first plane on one side thereof, to the first ground surface of the composite block, through an adhesive; and
grinding the side opposite to the first ground surface of the composite block to a given thickness where the surfaces of both the first and second optical materials are exposed.
The process of the invention facilitates the production control and designing as well as the control of the optical path lengths, and provides excellent composite optical elements.
A diffraction grating polarizer using a composite optical element according to the invention acts as a linear polarizer when the first optical element is made of a birefringent material and the second optical element is made of an isotropic material, and the component elements are combined so that a relation (ne1xe2x88x92no1)d=(M+1/2)xcex, in which d is the overall thickness of the resulting element, xcex is the wavelength of incident light, and M is an integer, holds between the refractive indexes no1, ne1 of the first optical element with respect to two natural linear polarizations, or ordinary light and extraordinary light, and the refractive index n2 of the second optical element.
The composite optical element is further combined with a Faraday rotator to make a composite optical element for optical isolator according to the invention. Thus the composite optical element for optical isolator formed from the optical element of the invention is a composite optical element comprising a Faraday rotator having a Faraday rotation angle of approximately 45xc2x0, first and second birefringent materials joined to one side of the rotator, and third and fourth birefringent materials joined to the other side of the rotator, wherein:
the light that has been transmitted through the first birefringent material passes through the third birefringent material;
the light that has been transmitted through the second birefringent material passes through the fourth birefringent material;
the optical axis of the first birefringent material and that of the second birefringent material intersect orthogonally;
the optical axis of the third birefringent material and that of the fourth birefringent material intersect orthogonally;
the optical axis of the first birefringent material and that of the third birefringent material make an angle of about 45xc2x0 with respect to each other;
the first and second birefringent materials have the same ground principal planes flush with each other on both sides of them;
the third and fourth birefringent materials have the same ground prinicpal planes flush with each other on both sides of them;
the first, second, third, and fourth birefringent materials are of the same material and have approximately the same thickness d, substantially satisfying the equation
2(noxe2x88x92ne)d=(M+1/2)xcex
where no is the refractive index of the birefringent material to ordinary light, ne is the refractive index of the birefringent material to extraordinary light, M is an arbitrary integer, and xcex is the wavelength of the light.
A polarization-insensitive optical isolator according to the present invention that utilizes the composite optical element fabricated in accordance with the present invention comprises a first optical waveguide, a first lens, the said composite optical element, a second lens, and a second optical waveguide arranged in the order in which they have just been mentioned.
Light outgoing in the forward direction from the end of the first optical waveguide is converted by the first lens to parallel beams, the first light that has been transmitted through the first birefringent material passes through the third birefringent material, the second light that has been transmitted through the second birefringent material passes through the fourth birefringent material, and, after the passage through the third and fourth birefringent materials, the first light and second light, producing no optical path difference regardless of the optical path length difference, are combined by the second lens into the second optical waveguide;
while light outgoing in the reverse direction from the end of the second optical waveguide is converted by the second lens to parallel beams, the third light that has been transmitted through the third birefringent material passes through the first birefringent material, the fourth light that has been transmitted through the fourth birefringent material passes through the second birefringent material, and, after the passage through the first and second birefringent materials, the third light and fourth light produce a half-wave optical path difference regardless of polarization and are not combined by the first lens into the first optical waveguide.
A polarization-insensitive optical isolator according to the invention comprises:
a composite optical element according to the present invention is inserted between a first single-mode optical waveguide and a second single-mode optical waveguide; and
of the light rays outgoing in the forward direction from the end of the first single-mode optical waveguide, the first ray that has been transmitted through the first birefringent material passes through the third birefringent material, the second ray that has been transmitted through the second birefringent material passes through the fourth birefringent material, and, after the passage through the third and fourth birefringent materials, the first and second rays, producing no optical path difference regardless of polarization, are combined into the second single-mode optical waveguide;
while, of the rays outgoing in the reverse direction from the end of the second single-mode optical waveguide, the third ray that has been transmitted through the third birefringent material passes through the first birefringent material, the fourth ray that has been transmitted through the fourth birefringent material passes through the second birefringent material, and, after the passage through the first and second birefringent materials, the third and fourth rays produce a half-wave optical path difference regardless of polarization and are not combined into the first single-mode optical waveguide.
The composite optical element of the present invention can also be utilized to make up an optical circulator. An optical circulator according to the present invention is based on a waveguide type Mach-Zehnder interferometer circuit which comprises two optical waveguides for input and output, two 3-dB directional couplers, and two optical paths of equal length held between the two directional couplers, and:
a composite optical element according to the present invention is inserted between the two optical paths;
the first and third birefringent material regions being located in one of the optical paths and second and fourth birefringent material regions in the other optical path.
A process according to the present invention for producing the composite optical element according to the present invention comprises the steps of:
forming a plurality of first grooves at predetermined intervals in a first birefringent material;
forming a plurality of second grooves at predetermined intervals in a second birefringent material;
bonding the first birefringent material having the first grooves and the second birefringent material having the second grooves together, with their grooves and lands staggered to fit each other, through an adhesive to provide a first composite block;
grinding one side of the composite block to a thickness where both surfaces of the first and second birefringent materials are exposed, thereby forming a first ground surface;
bonding a Faraday rotator at one plane thereof to the first ground surface of the first composite block;
grinding the side opposite to the first ground surface of the composite block to a given thickness where the surfaces of both the first and second birefringent materials are exposed;
forming a plurality of third grooves at predetermined intervals in a third birefringent material;
forming a plurality of fourth grooves at predetermined intervals in a fourth birefringent material;
bonding the third birefringent material having the third grooves and the fourth birefringent material having the fourth grooves together, with their grooves and lands staggered to fit each other, through an adhesive to provide a second composite block;
grinding one side of the composite block to a thickness where both surfaces of the third and fourth birefringent materials are exposed, thereby forming a second ground surface;
bonding a Faraday rotator at the opposite plane thereof to the second ground surface of the second composite block; and
grinding the side opposite to the second ground surface of the second composite block to a given thickness where the surfaces of both the third and fourth birefringent materials are exposed;
said first, second, third, and fourth birefringent materials being of the same material.
The composite optical elements according to the invention have broad applications for optical isolators, optical circulators, and optical switches.
The present invention provides by a composite optical element in which a first optical material and a second optical material are joined to one plane of a third optical material, the first and second optical materials being the same in thickness and having ground surfaces flush with each other on the same principal planes thereof. The invention also provides by the use of the composite optical element in fabricating various devices such as polarizers, optical isolators, optical circulators, and optical switches.
The invention is further provides a process for producing a composite optical element comprising the steps of:
forming a plurality of first grooves at predetermined intervals in a first optical material;
forming a plurality of second grooves at predetermined intervals in a second optical material;
bonding the first optical material having the first grooves and the second optical material having the second grooves together, with their grooves and lands staggered to fit each other, through an adhesive to provide a composite block;
grinding one side of the composite block to a thickness where both surfaces of the first and second optical materials are exposed;
bonding a third optical material, with a first plane on one side thereof, to the first ground surface of the composite block, through an adhesive; and
grinding the side opposite to the first ground surface of the composite block to a given thickness where the surfaces of both the first and second optical materials are exposed. The invention still further provides the use of the process in making various devices such as polarizers, optical isolators, optical circulators, and optical switches. The process facilitates the manufacture of composite optical elements and precision control of the process, rendering it possible to enhance the efficiency of production and the of the composite optical elements while reducing the cost of the elements and the optical devices utilizing the same.
Now, for a better understanding of the invention, the optical isolator according to the present invention that uses two composite optical elements according to the present invention will be explained along with the principle of operation of the isolator.
Referring to FIG. 1, the element is constructed so that the light that has passed through the first birefringent material travels through the Faraday rotator and thence the third birefringent material while the light that has passed through the second birefringent material proceeds through the Faraday rotator and thence the fourth birefringent material. The optical axes of the first and second birefringent materials intersect orthogonally and likewise the axes of the third and fourth birefringent materials intersect orthogonally. The optical axes of the first and third birefringent materials make an angle of approximately 45xc2x0 with respect to each other.
1) When light of linear polarization parallel to the optical axis of the first birefringent material is incident in the forward direction
This linearly polarized light is transmitted as extraordinary light (refractive index ne) through the first birefringent material. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as ordinary light (refractive index no) through the third birefringent material. Meanwhile the same incident light is transmitted as ordinary light (refractive index no) through the second birefringent material. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as extraordinary light (refractive index ne) through the fourth birefringent material. The optical path length for the passage of light through the first and third birefringent materials is (ne+no)d. Likewise the optical path length for passage through the second and fourth birefringent materials is (no+ne)d, where d stands for the thickness of the birefringent materials. Since the two optical path lengths are equal, the light emerges from the optical isolator straight forward without diffraction.
2) When light of linear polarization perpendicular to the optical axis of the first birefringent material is incident in the forward direction
This linearly polarized light passes as ordinary light (refractive index no) through the first birefringent material, as extraordinary light (refractive index ne) through the third birefringent material, as extraordinary light (refractive index ne) through the second birefringent material, and as ordinary light (refractive index no) through the fourth birefringent material. The optical path length for the passage of light through the first and third birefringent materials is (no+ne)d, and that for the passage through the second and fourth birefringent materials is also (ne+no)d. The equal length of the two optical paths allows the light to travel straight forward without diffraction.
Thus light in the forward direction proceeds straight forward regardless of polarization.
The two optical path lengths for linear polarization in 1) and 2) above being equal as (no+ne)d, it is possible to obtain an ideal optical isolator free from dispersion of polarization.
3) When light of linear polarization parallel to the optical axis of the third birefringent material is incident in the reverse direction
This linearly polarized light passes as extraordinary light (refractive index ne) through the third birefringent material, as extraordinary light (refractive index ne) through the first birefringent material, as ordinary light (refractive index no) through the fourth birefringent material, and as ordinary light (refractive index no) through the second birefringent material. The optical path length for the passage of light through the first and third birefringent materials is (no+no)d, and that for the passage through the second and fourth birefringent materials is likewise (ne+ne)d. The light diffracts when the optical path difference is set to a half wave, or to substantially satisfy the equation
2(noxe2x88x92ne)d=(M+1/2)xcexxe2x80x83xe2x80x83(1)
where M is an arbitrary integer and xcex is the wavelength of the light.
4) When light of linear polarization perpendicular to the optical axis of the third birefringent material is incident in the reverse direction
This linearly polarized light passes as ordinary light (refractive index no) through the third birefringent material, as ordinary light (refractive index no) through the first birefringent material, as extraordinary light (refractive index ne) through the second birefringent material, and as extraordinary light (refractive index ne) through the fourth birefringent material. The optical path length for the passage of light through the first and third birefringent materials is (ne+ne)d, and that for the passage through the second and fourth birefringent materials is likewise (no+no)d. The light diffracts when the optical path difference is set to a half wave, or to substantially satisfy the equation
2(noxe2x88x92ne)d=(M+1/2)xcexxe2x80x83xe2x80x83(1)
where M is an arbitrary integer and xcex is the wavelength of the light.
As described above, the optical isolator according to the present invention functions as a polarization-insensitive optical isolator, since it allows light in the forward direction to proceed straight without dependence upon polarization and diffracts light in the reverse direction again without dependence upon polarization.
In the optical isolator of the invention the first to the fourth birefringent materials are all of the same material, and the precision of their optical path difference is dictated by the precision of the thickness d. Under the invention, both surfaces of the first and second birefringent materials and bolt surfaces of the third and fourth birefringent materials are ground so as to ensure high precision of the thickness d of the regions between the two ground surfaces.
For the optical isolator of the invention the refractive indexes no and ne of the birefringent materials can be precisely measured before fabrication, and therefore the isolator is easy to design (including the setting of the thickness d) and the yield of the products is improved.
Since quartz, rutile, and other single crystal materials may be employed as the birefringent materials, outstanding resistance to environments is attained, and there is little possibility of the resulting optical isolator being deteriorated in performance due to changes in the refractive indexes with time.